Process for preparing cyclohexanedicarboxylic esters

Details Process for preparing cyclohexanedicarboxylic esters Process for preparing cyclohexanedicarboxylic esters

2002-06-03

2004-05-25

Killos, Paul J. (Department: 1625)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acid esters

Reexamination Certificate

active

06740773

ABSTRACT:

The present invention relates to a process for preparing cyclohexanedicarboxylic esters (hexahydrophthalic diesters) by esterifying benzenedicarboxylic acids or anhydrides of these with linear or branched aliphatic C
4
-C
14
monoalcohols, and then hydrogenating. The reaction of the aromatic dicarboxylic esters with hydrogen takes place in the presence of selected nickel catalysts in the gas or liquid phase.
STATE OF THE ART
Hydrogenated benzenedicarboxylic esters are used, inter alia, as intermediates in industrial organic chemistry and as plasticizers for plastics, for coating compositions, for sealing compounds, and in rubber products.
The effectiveness of the plasticizers derives from changes which they bring about in the physical properties of the highly polymerized thermoplastics, preferably using their solvating and swelling capability, but without reacting with the thermoplastics. Thermoplastic and plasticizer form a homogeneous system with a thermoplastic range shifted to lower temperatures than for the original polymer, the result being that the moldability and the elasticity of the polymer, for example, are increased and its hardness is reduced.
If plasticizers are to have a wide range of possible applications, they have to comply with a number of requirements. Ideally, they should be odorless, colorless, lightfast, low-temperature-resistant, and heat-resistant. In addition they are expected to undergo no change when exposed to moisture or water, and to be flame-retardant and have low volatility and, in particular, also to be toxicologically non-hazardous. In addition, their production should be simple and, in order to comply with environmental demands, avoid any formation of pollutant wastes, such as aqueous effluents comprising non-recyclable by-products and hazardous materials.
The compounds very widely used industrially as plasticizers include the esters of the isomeric phthalic acids, in particular esters which derive from ortho-phthalic acid and from plasticizer alcohols, i.e. linear or branched primary alcohols having from 4 to 14 carbon atoms in the molecule. They are used as single compounds or as mixtures of various esters. However, a factor which militates against their unrestricted use is that concerns have recently been raised as to whether this class of substances may be hazardous to health. They are therefore not permitted for use with food or drink, e.g. as a packaging material, or in other products whose use is subject to particular precautions for preventative healthcare reasons. Products of this type include, for example, items in day-to-day use, such as household articles and articles for childcare uses, including toys, and also products used in the medical sector. The plasticizers used for auxiliary or finished products made from thermoplastics and destined for these specific application sectors are not therefore phthalates, but compounds free from any toxic hazard. They include the esters of citric acid, whose application is limited for economic reasons to specific sectors, e.g. toys. Esters of cyclohexane di- and polycarboxylic acids (hexahydrobenzenedi- and -polycarboxylic acids) are much more freely available and therefore have potential for wider use.
The use of a cyclohexanedicarboxylic ester, namely 2-ethylhexyl cyclohexane-1,2-dicarboxylate, as plasticizer is known from DE-A 12 63 296. DE 199 27 977 A1 gives a comprehensive review of cyclohexanedi-, -tri-, and -tetracarboxylic esters which are suitable plasticizers for plastics. That publication in particular points out the advantageous toxicological properties of the compounds mentioned.
Cyclohexanedicarboxylic esters are usually prepared by catalytic hydrogenation of benzenecarboxylic esters. An example of a process of this type is the subject matter of DE 28 23 165 A1. The catalysts used in that procedure are in particular nickel but also the platinum metals ruthenium, rhodium or palladium, applied to a lithium aluminum spinel (LiAl
5
O
6
) support.
Another process for preparing cyclohexanedicarboxylic esters is described in DE 197 56 913 A1. In that publication, benzenedicarboxylic esters in the form of a single compound or in a mixture with other benzenedicarboxylic esters are reacted with a gas comprising hydrogen in the presence of a catalyst which encompasses at least one metal of the 8th transition group of the Periodic Table, alone or together with at least one metal of the 1st or 7th transition group of the Periodic Table, applied to a support which has macropores. Ruthenium in particular is used as catalytically active metal.
The known processes require the use of pure benzenedicarboxylic esters. The starting compounds therefore have to be subjected to complicated purification steps after their synthesis and prior to the reaction with hydrogen. The processes of the prior art also rely on the use of platinum metal catalysts, in particular ruthenium catalysts, for carrying out the hydrogenation. Reclamation of the catalysts has to be as quantitative as possible in order to restrict losses of precious metal over the entire reaction. The methods required for this purpose necessitate further steps in the reaction and therefore the provision of additional sections of plant. In these circumstances it is of interest to simplify the preparation process and to use appropriate conduct of the reaction to restrict the number of steps in the reaction to a minimum and thus increase the cost-effectiveness of the process.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a process which prepares cycloaliphatic dicarboxylic esters from the corresponding benzenedicarboxylic acids or anhydrides of these and which links the benzenedicarboxylic ester synthesis with the subsequent hydrogenation to give an integral process in such a way that separate steps for isolation and purification can be dispensed with.
THE INVENTION
According to the invention, the object described above is achieved by means of a process for preparing cyclohexanedicarboxylic esters by esteryfing of a benzenedicarboxylic acid or a benzenedicarboxylic anhydride with an aliphatic C
4
-C
14
monoalcohol in the presence of an acid or of a catalyst containing titanium, zirconium or tin, at temperatures of from 100 to 250° C., with removal of any water formed and neutralization of the reaction mixture once the esterification has been completed by adding an alkaline reagent, which comprises hydrogenating the neutralized esterification mixture where appropriate after removal of the water generated by the neutralization without other pretreatment in the presence of a nickel catalyst, and then isolating the cyclohexanedicarboxylic ester by separating off the by-products from the reaction mixture.
The novel process permits cyclohexanedicarboxylic esters to be prepared in successive steps of a reaction starting from benzenedicarboxylic acids or anhydrides of these. It should be particularly emphasized that isolation of intermediates is not necessary either for reasons of process technology or in connection with intermediate purification steps. Nevertheless, the products obtained from the reaction are highly pure esters of cycloaliphatic dicarboxylic acids and comply with all of the requirements which permit their successful use as plasticizers. In particular, the esters have excellent color properties, such as colorfastness, and extremely low conductivity, properties which give them a broad field of application in plastics processing.
The novel process starts from dicarboxylic acids or dicarboxylic anhydrides of benzene and from aliphatic monoalcohols having from 4 to 14 carbon atoms in the molecule as starting compounds. For the purposes of the invention, dicarboxylic acids of benzene are understood to be the positional isomers phthalic acid (o-phthalic acid), isophthalic acid (m-phthalic acid), and terephthalic acid (p-phthalic acid), in particular o-phthalic acid. The alcohol component of the esters may be either linear or branched compounds of the molecular size mentioned. Preference is given to alcohols hav

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